Chromium plating

ABSTRACT

An aqueous acidic hexavalent chromium electroplating bath containing sulfate and fluoride or complex fluoride catalysts and an effective microcracking amount of an organic compound of the class exemplified by ortho-benzoylsulfimide, known to the art as saccharin.

United States Patent [191 Brown et al.

[ 5] Feb. 18, 1975 CHROMIUM PLATING A Canada [73] Assignee: Oxy Metal Finishing Corporation,

Warren, Mich.

[22] Filed: Nov. 15, 1973 [21] Appl. No.: 416,124

[52] US. Cl. 204/51 [51] Int. Cl C23b 5/06 [58] Field of Search 204/51, 43 R, 44, 105 R [56] References Cited UNITED STATES PATENTS 2,195,409 4/1940 Flett 204/51 X 3,505,183 4/1970 Seyb et al. 204/5l FOREIGN PATENTS OR APPLICATIONS 811,882 4/1959 Great Britain 204/51 Primary ExaminerG. L. Kaplan Attorney, Agent, or FirmB. F Claeboe [57] ABSTRACT An aqueous acidic hexavalent chromium electroplating bath containing sulfate and fluoride or complex fluoride catalysts and an effective microcracking amount of an organic compound of the class exemplified by ortho-benzoylsulfimide, known to the art as saccharin.

7 Claims, No Drawings 1 CHROMIUM PLATING BACKGROUND OF THE INVENTION The importance of micro craze-cracking in the production of greatly improved corrosion protection with decorative nickel-chromium and copper-nickelchromium plate is well established, as can be seen from the publications by W. E. Lovell et al, Proc. Amer. Electroplaters Soc., Vol. 47, p. 215 (1960) and J. H. Lindsay, et. al., ibid, (1961), Vol.48, p. 165.

A significant problem which is associated with present decorative microcracked chromium deposits ob tained from acidic hexavalent chromium plating baths is that microcracking of the plate does not extend sufficiently down into the low current density areas, in the normal plating time of approximately 7 to 10 minutes required to develop effective and commercially accept able microcracking.

SUMMARY OF THE INVENTION The present invention relates generally to the electro-deposition of chromium from aqueous acidic hexavalent chromium solutions. More particularly, it is directed to the electro-deposition of chromium with an increased tendency to develop fine craze-cracking over a wide range of cathode current densities, through the use of certain organic compounds, as exemplified in Table II, hereinafter, dissolved in acidic hexavalent chromium plating baths.

A principal object of this invention is to provide a method of obtaining microcracked chromium plate over a wide plating range with a minimum thickness of the chromium. It has been found by applicants that certain organic compounds(Table I), as exemplified by -obenzoyl sulfimide (saccharin), when added to acidic hexavalent chromium plating baths containing both sulfate and fluoride, or complex fluoride anions, as catalysts make possible the development of extensive microcracking down into the middle and low current density areas in decorative chromium plating.

DESCRIPTION OF PREFERRED EMBODIMENTS The present invention is directed to the electrodeposition of chromium from an aqueous acidic hexavalent chromium electroplating bath containing an effective microcracking amount of a compound of the general composition set forth in Table I appearing hereinafter. By microcracking is meant that the chromium deposit has from about 300 to about 3,000 cracks per linear inch.

The following example illustrates the improvements which can be obtained in increasing the extent of microcracking in a decorative chromium plate by the use of saccharin, which is one of the most effective compounds of Table I dissolved in the bath. A chromium plating bath (Bath A) of the following composition:

2 g/l Bath Temperature lF (l05l30F) generally S shaped steel cathode with square sides rather than rounded sides and with a tab remaining at the upperside of the S is formed from a cold-rolled steel strip about 9 inches in length and approximately 1.25 inches in width.

The strip is cleaned and bright nickel plated for 15 minutes using about 40 amps/sq.ft. on the panel, rinsed, and then inserted between two lead anodes in about three liters of the chromium plating bath maintained at can be used for a comparison of the results obtained about F. Using 15 amperes on the panel (average of about amp/sq.ft.) and 10 minutes plating time, the microcracking generally appears only on a narrow section of the high current density areas, that is, near the edges, with some spotty cracking toward the middle current density areas. However, upon the addition of 0.5 to 1.0 g/l of saccharin, dense microcracking resulted on all but the most deeply recessed areas of the panel. The number of cracks per linear inch measured in various directions on substantially all of the panels ran from about 800 to 1,200, with substantially all of the craze-cracking averaging about 1,000 cracks per linear inch. Thermal shock, such as a dip in relatively hot water at a temperature between to 200F for l to 2 minutes, or the usual final hot water rinse to aid drying, aids in the rapid development of microcracking. A highly stressed underneath nickel plate was not .used, even though this further increases the microcracking tendency and makes possible the use of thinner chromium plate and yet one still obtains extensive microcracking. The underneath bright nickel plate used in the above test had essentially zero stress. The Dubpernell test, that is, about a l0 minute plate at low current density from an acid copper bath applied to the chromium plate was used to identify the extent of microcracking. The chromium surface was photographed at 100 X and the extent and number of microcracks per linear inch were determined.

Instead of using sodium silicofluoride as the sole source of the silicofluoride anion, the potassium salt or other silicofluoride salts can be used to furnish a similar concentration of silicofluoride anion. Also, saturated solutions of slightly soluble silicofluoride salts can be used, such as those of lanthanum, praseodymium or neodymium or mixtures thereof, and the balance of silicoflurodie anion can be provided by relatively small additions of sodium or potassium silicofluoride. In this regard, saturated concentrations of ceric and/or cerous silicofluoride salts of limited bath-solubility are excellent, and it is necessary to add relatively small concentrations of sodium or potassium silicofluoride for optimum total concentrations of silicofluoride anion with respect to the low concentrations of sulfate anion for attaining a maximum cathode current density range of decorative microcracked chromium when Saccharin is present. This appears in the formulation of Bath B below. Saccharin should be present in these baths in an optimum concentration of about 0.5 to about 1.0 gram/liter in order to obtain an extensive and dense microcracking. It is also within the contemplation of this invention to control the concentation of silicofluoride anion by using a saturated solution of potassium silicofluoride and then suppressing the ionization of this salt by use of relatively high concentrations of potassium ions derived from high concentrations of potassium dichromate added during the making up of the acidic hexavalent chromium plating bath. In substitution for silicofluoride or fluoride anions, there may be used chromate or dichromate added in order to repress the;

ionization of the strontium sulfate. 1f the fluoride or complex fluoride ion is decreased in concentration,

then the degree of microcracking is decreased and yet the compounds of Table I are still useful for purposes of extending the degree of microcracking. If the fluoride or complex fluoride anions are not used and the chromic acid to sulfate ratio is decreased to 150 to 1 or to 100 to 1 or 75 to 1, then for decorative chromium plating where the average thickness of chromium is normally kept to about 0.1 mil as a maximum, then the use of the compounds of Table 1 do not cause microcracking. However, the compounds of Table I are useful for slightly increasing the extent of microcracking with relatively more thick non-decorative uses of chromium plate, as for example, for engineering applications. In the latter uses, as for a hard chrome plate of about 0.3 mil to about 20 mils thickness used for wear resistance, macro'crazecracks of about to 200 cracks per linear inch can serve to retain traces of oil and in this manner thereby decrease wear in applications involving wear surfaces such as on cylinder liners or on the epitrochoidal track of Wankel rotary engine housing.

The optimum concentration for the best of the above Table 1 compounds, o-benzoyl sulfimide (saccharin) in the decorative microcracked chromium plating baths is about 0.5 to 1.0 g/l, for most of the others, somewhat higher concentrations such as 1.0 to 1.5 g/l may be found to be necessary. Actually, concentrations as high as 10 g/l, in fact up to saturation concentrations, can be used in the bright decorative chromium plating baths. The compounds of Table 1 may be added to the baths as free acids or as salts, such as the strontium, calcium, lithium, sodium or potassium salts. In any case, in a relatively strong acid bath, the compounds are present predominantly as acids. Technical grades can of course be used.

Where thick chromium plate is used in engineering applications as for hard chrome plating, then concentrations of the compounds of Table l as low as about 0.05 g/l are helpful for slightly increased macrocrackmg.

Mixtures of the compounds of Table I can be used, in the manner of Bath B below. The compounds of Table I are compatible with the anti-spray surfactants described in U.S. Pat. No. 2,750,334 and also with those described in U.S. Pat. No. 3,432,408.

The compounds of Table I function effectively in chromium plating baths comprising a ratio of about 300 to 1, and 250 to 1 of chromic acid to sulfate with the same fluoride concentrations shown for Bath A or even with slightly higher fluoride concentrations. However, at the chromic acid concentration shown in Bath A hereinbefore set forth andat the concentration of fluoride or complex fluoride shown therein, the optimum ratio of the chromic acid to sulfate is approximately 200 to 1. When the ratio is as high as 300 to 1 in Bath A, there is a tendency for haziness to occur in the high current density areas where the plate is relatively thicker. Also with higher ratios of the order of 300 to 1 of chromic acid to sulfate, the concentration of silicofluoride or other fluoride ion is preferably increased by about 0.5 g/l. With higher fluoride or complex fluoride concentrations and lower bath temperatures approximating -l 15F, the microcracking tendency is increased. However, with the lower bath temperatures, the high current density areas have a more marked tendency for hazy appearance. Very small concentrations, as for example, 0.1 to about 10 mg/l of selenious oxide or the equivalent concentration of selenious or selenic acid when added to baths of type A or B help the microcracking, both as to density and extent, when only about 0.1 or about 0.2 g/l of the Table 1 compounds are used in these baths. When the optimum quantity of about 0.5 to 1.0 g/l of the Table I compounds such as saccharin compounds are present in the baths of types A and B, then as little as 0.1 to about 2 mg/l of Se(),, added to the chromium baths like A and B will make it possible to obtain a very high density of microcracking of the order of about 800 to 1,200 microcracks per linear inch over a relatively wide current density range, from the highest down to very close to the lowest, even with a decrease of about 0.5 g/l of the sodium silicofluoride concentration. Thus, Bath B would become Bath C below, which yields decorative chromium plate with a very extensive and high density microcracking, but with a faint bluish haze or cast.

The blue haze in the chromium plate from Bath C is not as intense as when about 5 to mg/l of SeO is added to chromium plating baths, and with concentrations of SeO above 10 mg/l,the blue haze in the decorative chromium plate may be sufficiently noticeable that in many cases it may not be acceptable for decorative plating. The selenium is added as selenious oxide, selenious or selenic acids or the salts of the acids, or added as sodium or other selenocyanates, or added as a seleno organic compound such as selenourea, N, N- dimethyl selenourea, triphenyl selenium chloride, etc. Telluric acid does not normally cause the blue haze, but is very much less effective than selenious oxide or selenious or selenic acid in causing or aiding microcracking even when used in concentrations as high as 0.5 to even 2 or 5 g/l.

The rate of consumption of the Table 1 compounds, as exemplified by the sodium salt of saccharin is about 0.4 g/l or 1.6 grams per gallon for 1,000 amp. hours per gallon.

The reason extensive microcracking of the decorative chromium deposit makes possible greatly improved corrosion protection with nickel-chromium and copper-nickel-chromium plate is generally as follows. With highly porous chromium plate, due to the extensive microcraking, there results, in the presence of a rather corrosive environment, a substantially increased number of small anodic areas, and this causes a greatly diminshed anodic or corrosion current density in the nickel exposed in the microcracks of the chromium, and consequently the rate of pitting penetration through the nickel is very markedly decreased.

1f steel panels are plated with a half mil of bright copper and one mil of bright nickel, and chromium plated with 0.03 to 0.05 mil ofthe usual decorative chromium plate, and these panels are compared to similar steel panels plated with generally the same copper and nickel plate, but with microcracked chromium of the same thickness as the control, the difference in corrosion protection to the steel on exposure in a marine or industrial atmosphere is striking.

As already mentioned, about 2 g/l is the maximum concentration needed for the compounds of Table 1, though even 10 g/l or even saturation concentrations which are, in general, about 15 g/l as the free acid, can be used, though such high concentrations are normally not necessary and are actually relatively wasteful in most cases.

It should be pointed out that when the chromium plating thickness is increased above about 0.03 mil to about 0.05 to 0.1 mil in thickness, then the degree or the number of cracks per linear inch and the extent of microcracking into the lower current density areas is increased for baths like A, B and C above. For baths like C, it is then not necessary to use even a trace of selenious acid to increase the extent of microcracking when the silicofluoride catalyst concentration is only that obtained from saturation concentrations of cerium silicofluorides in the acidic hexavalent chromium plating bath. This is especially true when the bath temperatures are kept around 1 15F.

The hydrolytic products of saccharin and substituted saccharins, also function by themselves or together with the compounds of Table l to increase microcracking. These hydrolytic products and similar or related compounds are shown in Table 11.

The compounds of Table l and Table 11 may be broadly classified as bath-soluble substituted benzene sulfonamides and their hydrolytic products.

It should be emphasized that optimum bath temperatures are about 125F, and preferred chromic acid concentrations are about -250 grams/liter. The optimum ratios of chromic acid to sulfate are about 150 to 300 to l and the preferred fluoride or complex fluoride concentration is from 0.3 to 3 g/l, calculated as silicofluoride ion. The sulfate ion may be present in a concentration of about 0.5 to about 4 g/l, and the chromic acid concentration may be as high as 400 g/l. However, the optimum ratios and concentrations of the inorganic components of the acidic hexavalent chromium plating baths in order to obtain maximum microcracking for plating times of about 5 to 10 minutes have already been listed, for example in U.S. Pat. No. 3,408,272.

The organic compounds of Tables 1 and 11 present in these chromium plating baths decrease the plating time needed to obtain maximum microcracking and also extend the microcracking farther down into the lower current density ranges.

What is claimed is: p

1. An aqueous acidic hexavalent chromium electroplating bath containing dissolved therein an organic compound selected from the class of carbonyl substituted benzene sulfonamides and their organic hydrolytic products in concentrations of about 0.] to about 10 grams per liter to cause an increase in crazecracking tendency of the chromium plate derived from the bath.

2. A bath as defined in claim 1, wherein the organic compound is o-benzoyl sulfimide.

3. A bath as defined in claim 1, wherein the organic ganic compound is sulfobenzoic acid. 

1. AN AQUEOUS ACIDIC HEXAVALENT CHROMIUM ELECTROPLATING BATH CONTAINING DISSOLVED THEREIN AN ORGANIC COMPOUND SELECTED FROM THE CLASS ON CARBONYL SUBSTITUTED BENZENE SULFONAMIDES AND THEIR ORGANIC HYDROLYTIC PRODUCTS IN CONCENTRATIONS OF ABOUT 0.1 TO ABOUT 10 GRAMS PER LITER TO CAUSE AN INCREASE IN CRAZE-CRACKING TENDENCY OF THE CHROMIUM PLATE DERIVED FROM THE BATH.
 2. A bath as defined in claim 1, wherein the organic compound is o-benzoyl sulfimide.
 3. A bath as defined in claim 1, wherein the organic compound is a chlorosaccharin.
 4. A bath as defined in claim 1, wherein the organic compound is sulfobenzoic acid.
 5. A method of depositing chromium comprising passing an electric current from an anode to a cathode through the electrolyte of claim 1 for a period of time sufficient to form a microcracked chromium deposit.
 6. A method as defined in claim 5, wherein the organic compound is a chlorosaccharin.
 7. A method as defined in claim 5, wherein the organic compound is sulfobenzoic acid. 